The present inventions relate generally to a laboratory or portable measurement method and system, and more particularly, to a method and system for the point of sale measurement of the water and sediment content in a petroleum sample.
Conventional Methods for Measuring Water Content
A determination of water content in crude oil is required to measure accurately net volumes of actual oil in sales, taxation, exchanges, and custody transfers. The water content of crude oil is also significant because it can cause corrosion of equipment and problems in processing. Thus, various methods have been developed for measuring the water content of crude oil.
Karl Fischer Titration Method
In 1935, German scientist, Karl Fischer, developed a titrimetric determination of water content using a reagent that contained iodine, sulphur dioxide, anhydrous pyridine and anhydrous methanol. This method can be subdivided into two main techniques: volumetric titration and coulometric titration.
The volumetric technique involves dissolving the sample in a suitable solvent and adding measured quantities of a reagent containing iodine until an end point is reached. This end point is determined potentiometrically using a platinum electrode. When all of the water has reacted, the platinum measuring indicator electrode will electronically instruct the burette to stop dispensing. The volume of KF reagent dispensed is recorded. Based on the concentration of iodine in the KF reagent, the amount of water present is then calculated.
However, even with the automatic or semi-automatic instruments commercially available, there are certain problems associated with this technique. These problems include long analysis time, required reagent calibration, and high reagent consumption rate.
In the coulometric technique developed by Meyer and Boyd in 1959, the sample is introduced into a mixture of pyridine/methanol that contains iodide ions and sulphur dioxide. The electrode system consists of an anode and cathode platinum electrodes that conduct electricity through the cell. Iodine is generated at the anode and reacts with any water present. The production of iodine is directly proportional to the amount of electricity according to Faraday's Law as shown in the equation:2I−−2e→I2.
According to the stoichiometry of the reaction, 1 mole of iodine will react with 1 mole of water, and combining this with coulometry, 1 milligram of water is equivalent to 10.71 coulombs of electricity. Therefore, it is possible to directly determine the amount of water present in a sample by measuring the electrolysis current in coulombs. The platinum indicating electrode voltametrically senses the presence of water and continues to generate iodine until all the water in the sample has been reacted.
From this titration, the onboard microprocessor calculates the total amount of current consumed in completing the titration and the time to completion in seconds. Based on the relationship between coulombs and iodine, the exact amount of iodine generated is recorded. Since water reacts in the 1:1 ratio with iodine, the amount of water can be calculated.
Although the original Karl Fischer reagent contained pyridine, most reagent manufacturers now use other amines such as imidazol.
Karl Fischer titration is one of the most widely used techniques for measuring the water content in a large range of samples. However, it has limits that affect its usefulness. For example, it utilizes hazardous reagents that require the operator to exercise care in the storing, handling, and disposing of reagents that degrade with time. With the techniques, a total sample size of 0.5 ml. or smaller is taken from a larger sample size, typically 250 ml. The small sample size utilized by the techniques causes errors and cannot measure water percentages over 1% accurately. Also, the Karl Fischer titration techniques are operator intensive and do not provide any information with regard to the amount of sedimentation in a sample.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.7—Standard Test Method for Water in Crude Oils by Potentiometric Karl Fischer Titration and Chapter 10.9—Determination of Water in Crude Oils Coulometric Karl Fischer Titration for the complete protocols, which are hereby incorporated by reference.)
Centrifuge Method
In the standard method for determining the water content in crude oil by centrifuge, equal volumes of a sample and water saturated toluene are placed into two cone-shaped centrifuge tubes. The tubes are then corked and placed into a centrifuge. The tubes are then spun for 10 minutes at a minimum relative centrifugal force of 600 calculated from the following equation:rmp=1335√{square root over (rcf/d)}where:                rcf=relative centrifugal force and        d=diameter of swing measured between tips of opposite tubes when in rotating position, mm.        
Immediately after the centrifuge comes to rest following the spin, the combined volume of water and sediment at the bottom of each tube is read and recorded. The spin is then repeated until the combined volume of water and sediment remains constant for two consecutive spins. The final volume of water is then recorded for each tube.
The standard method for determining the water content by centrifuge is not entirely satisfactory. The amount of water detected is almost always lower than the actual water content. Therefore, when a high accurate value is required, another method must be used. This method also requires hazardous solvents and has very poor accuracy and reproducibility.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.3—Standard Test Method for Water and Sediment in Crude Oil by the Centrifuge Method (Laboratory Procedure) for the complete protocol, which is hereby incorporated by reference.)
Distillation Method
In the standard test for determining the water content by distillation, the sample is heated under reflux conditions with a water immiscible solvent that co-distills with the water in the sample. The condensed solvent and water are continuously separated in a trap wherein the water settles in the graduated section of the trap, and the solvent returns to the distillation flask. The amount of water can then be determined on a volume or a mass basis.
The precision of this method can be affected by water droplets adhering to surfaces in the apparatus and, therefore, not settling into the water trap to be measured. To minimize this problem, all apparatus must be chemically cleaned at least daily to remove surface films and debris that hinder the free drainage of water in the apparatus.
If the system forms azeotropes, as in a benzene and cyclohexane system, a different problem arises, —the azeotropic composition limits the separation, and for a better separation, this azeotrope must be bypassed in some way. At low to moderate pressure, with the assumption of ideal-gas model for the vapor phase, the vapor-liquid phase equilibrium (VLE) of many mixtures can be adequately described by the following Modified Raoult's Law:yiP=xiγiPisat for i=1, . . . ,cwhere                yi=mole fraction of component i in vapor phase;        xi=mole fraction of component i in liquid phase;        P=system pressure;        Psat=vapor pressure of component i; and        γi=liquid-phase activity coefficient of component i.        
When γi=1, the mixture is said to be ideal, and the equation simplifies to Raoult's Law. Nonideal mixtures (γi≠1) can exhibit either positive (γi>1) or negative deviations (γi<1) from Raoult's Law. In many highly nonideal mixtures, these deviations become so large that the pressure-composition (P−x, y) and temperature-composition (T−x, y) phase diagrams exhibit a minimum or maximum azeotrope point. In the context of the T−x, y phase diagram, these points are called the minimum boiling azeotrope (where the boiling temperature of the azeotrope is less than that of the pure component) or maximum boiling azeotrope (the boiling temperature of the azeotrope is higher than that of the pure components). About 90% of the known azeotropes are of the minimum variety. At these minimum and maximum boiling azeotrope, the liquid phase and its equilibrium vapor phase have the same composition, i.e.:xi=yi for i=1, . . . ,c  (2)
Two main types of azeotropes exist, i.e. the homogeneous azeotrope, where a single liquid phase is in the equilibrium with a vapor phase; and the heterogeneous azeotropes, where the overall liquid composition, which forms two liquid phases, is identical to the vapor composition. Most methods of distilling azeotropes and low relative volatility mixtures rely on the addition of specially chosen chemicals to facilitate the separation.
The drawbacks to this method include, for example, the fact that it utilizes hazardous solvents and produces hazardous vapors. This method also takes 2 to 3 hours to complete, and as with most distillation techniques, the accuracy and precision of the results will depend upon the skill of the technician performing the distillation. This method also does not provide any information with regard to the amount of sedimentation in the sample.
(Please see Manual of Petroleum Measurement Standards, Chapter 10.2—Standard Test Method for Water in Crude Oil Distillation for the complete protocol, which is hereby incorporated by reference.)
Zeolite Molecular Sieves
Molecular sieves, as used in this specification, include any material that can effectively be used to sequester or restrain or retain molecules in a material, such as, but not limited to, water molecules in a non-aqueous liquid, whether by physical capture within a crystalline structure, absorptive properties, adsorption, hydrogen bonding, or other means.
One class of molecular sieves includes crystalline, hydrated metal aluminosilicates. The commercially important types of molecular sieves are synthetically made, but their structure is similar enough to naturally occurring minerals to be classified as zeolites. Although the crystal structures of some of the molecular sieves are quite different, their absorbent property derives from their crystalline structure.
The crystalline metal aluminosilicate molecular sieves have a simple polyhedra arrangement. Each polyhedron is a three-dimensional array of (Si, AlO4) tetrahedral. The tetrahedra are formed by four oxygen atoms surrounding a silicon or aluminum atom. Each oxygen atom has two negative charges, and each silicon atom has four positive charges. This structure permits a net sharing arrangement, building a tetrahedron uniformly in four directions. The trivalency of aluminum causes the alumina tetrahedron to be negatively charged, requiring an additional cation to balance the system. Thus, the final structure has sodium, potassium, or calcium cations in the network. These “charge balancing” cations are the exchangeable ions of the zeolite structure.
Zeolites, one class of molecular sieves, exhibit electrical conductivity of an ionic type due to the migration of cations through the channel structure. The ability of the cations to carry a current depends upon their ionic mobility, charge, size, and location in the structure. The addition of water molecules to a dehydrated zeolite structure produces a pronounced change in the electrical conductivity of the zeolite. The conductivity of the zeolite increases with the amount of water present. The activation energy for conduction decreases with increasing adsorption of water. The influence of water is different for different zeolites. In some cases, the activation energy for conduction in a zeolite containing divalent ions is approximately twice that of a zeolite containing univalent ions.
When formed, this crystalline network is full of water, but with moderate heating, the moisture can be driven from the cavities without changing the crystalline structure—leaving countless cavities with their tremendous combined surface area and pore volume available for the adsorption of water or other materials.
With their large surface area and pore volume, molecular sieves then can perform virtually all the adsorption duties presently carried out by other absorbents. In addition, molecular sieves allow for a new dimension in process control because the pores of the crystalline network are uniform rather varied. Therefore, molecular sieves are able to differentiate molecules on the basis of molecular size and configuration.
Hence, molecular sieves utilize two adsorption mechanisms. They exhibit the capillary condensation phenomenon as a result of their large surface area and pore volume, and their polar surfaces have an electrostatic attraction for polar molecules such as water. This allows molecular sieves to be stronger absorbents than silica gel or alumina.
Another advantage to molecular sieves is that they can be packaged in foil-sealed bags to prevent moisture adsorption. This allows them to have long term stability and makes them easy to use. Also, the measured quantity of molecular sieves can be accurately controlled.
Although this application refers to the adsorptive properties and activities of molecular sieves, it understood that a certain amount of absorption also takes place. Therefore, for the sake of simplicity, references to the adsorptive properties and activities of molecular sieves are intended to include any absorptive properties and activities as well.
The “Load-Pulled” Effect
It is well known to electrical engineers generally (and particularly to microwave engineers) that the frequency of an RF (radio frequency) oscillator can be “pulled” (i.e. shifted from the frequency of oscillation that would be seen if the oscillator were coupled to an ideal impedance-matched pure resistance) if the oscillator sees an impedance that is different from the ideal matched impedance. Thus, a varying load impedance may cause the oscillator frequency to shift.
The present application sets forth various innovative methods and systems that take advantage of this effect. In one class of embodiments, an unbuffered RF oscillator is loaded by an electromagnetic propagation structure that is electromagnetically coupled, by proximity, to a material for which real time monitoring is desired. The net complex impedance seen by the oscillator will vary as the characteristics of the material in the electromagnetic propagation structure vary. As this complex impedance changes, the oscillator frequency will vary. Thus, the frequency variation (which can easily be measured) can reflect changes in density (due to bonding changes, addition of additional molecular chains, etc.), ionic content, dielectric constant, or microwave loss characteristics of the medium under study. These changes will “pull” the resonant frequency of the oscillator system. Changes in the medium's magnetic permeability will also tend to cause a frequency change since the propagation of the RF energy is an electromagnetic process that is coupled to both electric fields and magnetic fields within the transmission line.
For further background and information on load-pulled systems, the reader is referred to U.S. Pat. No. 6,630,833 to Scott, which is hereby incorporated by reference.
Other Approaches to Electrical Characterization
Various types of apparatus have been proposed for measuring the concentration of one substance in another, particularly the concentration of a liquid or flowable substance in another liquid or flowable substance. Various devices that utilize the broad concept of determining composition of matter by measuring changes in a microwave signal are disclosed in U.S. Pat. Nos. 3,498,112 to Howard; 3,693,079 to Walker; 4,206,399 to Fitzky et al.; 4,311,957 to Hewitt et al.; 4,361,801 to Meyer et al.; 4,240,028 to Davis Jr.; 4,352,288 to Paap et al.; 4,499,418 to Helms et al.; and 4,367,440 and 4,429,273, both to Mazzagatti; all of which are hereby incorporated by reference.
Although various systems utilizing microwave transmissivity or signal alteration characteristics have been proposed in the prior art, certain considerations in utilizing microwave energy to detect the presence of the concentration of one medium in another have not been met by prior art apparatus. In particular, it is desirable in certain instances to be able to accurately measure, on a continuous basis, the concentration or change in concentration of one fluid in another and particularly where the concentration of one fluid is a very low percentage of the total fluid flow rate or fluid mixture quantity. It is also desirable that the signal change caused by the presence of one substance or medium in another be easily measured and be relatively error free, again, particularly in instances where measurements of low concentrations of one substance such as a fluid in another substance such as another fluid are being taken. Moreover, it is important to be able to transmit the microwave signal through a true cross section of the composition being sampled or measured to enhance the accuracy of the measurement.
Typical systems for capacitive-based measurement have a capacitive element, used for parameter determination, as part of the resonant feedback loop around an active device. This method works well with very low-loss systems, but oscillation ceases with even slightly lossy measurements. As the frequency is increased into the microwave region, it becomes difficult to configure the resonant feedback loop due to the increase in loss versus frequency and the wavelength becoming comparable to the path length. In this case, the frequency is changed directly by the resonance change in the feedback loop, which includes the element that consists of the sample to be measured. This frequency change is limited to the characteristics and loss of the feedback path and can only be changed over a narrow frequency range with out cessation of oscillations. This limits the measurement technique to small samples of very low loss.
At higher frequencies (above approximately 100 MHz), the capacitive measurement technique fails to work, due to line lengths and stray capacitances. At such frequencies, resonant cavity techniques have been employed. (For example, a sample is placed in a resonant cavity to measure the loss and frequency shift with an external microwave frequency source that can be swept across the resonance with and without the sample in the cavity.) This method uses a highly isolated microwave frequency source that is forced by the user (rather than being pulled by the changing resonance) to change its frequency. This technique too meets substantial difficulties. For example, the use of multiple interfaces without a microwave impedance match at each interface causes extraneous reflections, which tend to hide the desired measurement data. This technique too gives errors with very lossy material, but in this case, it is due to the very rounded nature of the resonance curve (which is due to the low Q of the loaded cavity). This rounded curve makes it difficult to determine both the center frequency and the 3 dB rolloff frequency closely enough to be accurate in the measurement.
Another technique that is used encompasses the use of a very sharp rise time pulse to obtain time domain data from which frequency domain values are then derived through transformation techniques.
In U.S. Pat. No. 4,396,062 to Iskander, entitled “Apparatus and Method for Time-Domain Tracking of High-speed Chemical Reactions”, the technique used is time domain reflectometry (TDR). This contains a feedback system comprising a measurement of the complex permittivity by TDR means which then forces a change in frequency of the source, which is heating the formation to optimize this operation. Additionally, it covers the measurement of the complex permittivity by TDR methods.
U.S. Pat. No. 3,965,416 to Friedman appears to teach the use of pulse drivers to excite unstable, bi-stable, or relaxation circuits, and thereby propagate a pulsed signal down a transmission line that contains the medium of interest. The pulse delay is indicative of the dielectric constant of the medium. As in all cases, these are either square wave pulses about zero or positive or negative pulses. The circuit is a pulse delay oscillator where the frequency determining element is a shorted transmission line. The frequency generated is promoted and sustained by the return reflection of each pulse. The circuit will not sustain itself into a load that is lossy since the re-triggering will not occur without a return signal of sufficient magnitude. In addition, the circuit requires a load that is a DC short in order to complete the DC return path that is required for re-triggering the tunnel diodes.
The frequencies of operation of any pulse system can be represented as a Fourier Series with a maximum frequency that is inversely dependent upon the rise time of the pulse. Therefore, the system covered in the Friedman patent is dependent upon the summation of the frequency response across a wide bandwidth. This causes increased distortion of the return pulse and prevents a selective identification of the dielectric constant versus frequency. This also forces a design of the transmission system to meet stringent criteria to prevent additional reflections across a large bandwidth.
The low frequency limit of the TDR technique is determined by the time window, which is a function of the length of the transmission line. The upper extreme is determined by the frequency content of the applied pulse. In the case of this pulse delay line oscillator, the upper frequency is determined to a greater extent by the quality of impedance match (the lack of extra reflections) from the circuit through to the substance under study. These extra reflections would more easily upset the re-triggering at higher frequencies.
In one case (FIG. 1 of Friedman), the return reflection initiates a new pulse from the tunnel diode and, therefore, sets up a frequency (pulse repetition rate) as new pulses continue to be propagated. This is in essence a monostable multivibrator with the return reflection being the trigger. The problem implied, but not completely covered with this approach, is that due to the delay in pulses, the pulse train can overlap and cause multiple triggers to occur. These are caused by the re-reflections of the original parent pulse. An additional problem is with very lossy dielectrics, which will not provide enough feedback signal to initiate the next pulse. If the dielectric medium is of high enough dielectric constant to contain more than one wavelength, or if the dielectric constant of the samples vary greatly, multiple return reflections will alter the behavior of the circuit to render it useless due to the interfering train of return and parent pulses.
FIG. 3 of Friedman shows a bistable multivibrator that senses the return pulse by sampling and feeding back enough phase-shifted voltage to re-set the tunnel diodes. Since this device is also dependent upon the return to trigger or re-trigger the parent pulse, it suffers problems with lossy dielectrics and high dielectric constant mediums.
To overcome these problems, the relaxation oscillator of FIG. 4 of Friedman was proposed that contains a RC (resistor/capacitor timing) network that will maintain the generation of pulse trains using resistor 76 and capacitor 78 with the dielectric-filled transmission line affecting the regeneration of the pulses as the reflected parent pulse voltage is returned. Since the RC time constant is defining the basic repetition rate, some improvement is obtained in reducing second order effects. The transmission line is still an integral part of the overall relaxation oscillator, and lossy dielectrics may cause irregular circuit response. The proposed inverting amplifier as the pulse generator will not function at above approximately 1 MHz in frequency due to the characteristics of such inverting amplifiers. The tunnel diode can pulse up to a 100 MHz rate.
By contrast, the innovative system embodiments disclosed in the present application and its parents differ from the known prior art in using a microwave frequency generated by a free-running sine wave oscillator. The preferred oscillator has the versatile capability to work into a wide variety of transmission lines or other load impedance without generation of spurious data or cessation of oscillations. It will continue to oscillate with very lossy dielectrics. It is not a relaxation oscillator or a multivibrator. The frequency of the un-isolated oscillator is dependent upon the net complex impedance of the transmission line and will work into an open circuit as well as a short circuit. The net complex impedance at the frequency of operation of the oscillator looking at the transmission line containing the medium of interest results in stable oscillations through pulling of the unisolated oscillator. Only one frequency at any one time is involved in the disclosed system proposed (not counting harmonics that are at least 10 dB down from the fundamental). This provides for well-defined information and eases the transmission design criteria. This also provides for evaluation of the dielectric constant versus frequency that can improve resolution of constituents or ionic activity.
Another important difference from prior art is the separation of the load of interest from the resonant circuit proper. The configuration used isolates the two through the transistor. It is the non-linear behavior of the transistor that provides the changes in frequency as the load is changed. The loop gain of an oscillator must be unity with 180° phase shift. The initial gain of the transistor must be greater before oscillations begin in order for the oscillator to be self starting. This extra gain is reduced to unity by the saturation of the active device upon establishment of the oscillatory frequency. Saturating a device changes the gain (and accordingly the phase since it is non-linear) to maintain oscillations as the load changes. This will continue as the load changes as long as the transistor has appropriate phase and available gain to satisfy oscillations.
Aluminum Oxide for Moisture Adsorption
The use of aluminum oxide for moisture adsorption is well known in the industry. The surface attracts and retains water molecules by association with the bonds. Since this is a weak attraction, there is a point at which the absorption and desorption reaches an equilibrium with the surrounding moisture content. Moisture measurements have been made with capacitance measurements using a very thin aluminum oxide surface with imbedded electrodes. When the water is absorbed, the capacitance changes, and therefore, a measurement is made. This surface must be thin in order to allow the water molecules to accumulate in a region where the electrical field is present.
Moisture and Sediment Analysis
The present inventions describe systems and methods for the determination of the water and sediment content in a petroleum sample.
The present innovations include, in one embodiment, collecting a sample to be tested in a field bottle. The sample from the field bottle is then transferred from the field bottle, and into and through an analysis bottle containing a desiccant material. As the sample is being pulled through the analysis bottle, a microwave measurement system (or other scattering parameter measuring system) is used to measure the effects of the sample on the scattering parameters of the desiccant material. By measuring the effects of the sample on the scattering parameters of the desiccant material, the sample's moisture content can be determined. The sample's moisture can also be determined by measuring the expanded volume of the desiccant. A filter section having a sight glass with graduations is used to determine the sediment content of the sample.
Hence, the disclosed innovations provide a simple approach to measuring the moisture and sediment content in crude oil that is extremely fast, accurate, and reproducible without the use of hazardous chemicals. Other embodiments are described more fully below.